Traditionally, surveillance systems use off the shelf imagers for acquiring a video image. Typically these imagers generally are not small and require an external power supply. Also, these systems generally do not provide clear images if the captured video has a dark foreground with a bright background or visa versa. When such a video image is viewed on a monitor, little information can be extracted from it.
Furthermore, there are various types of imagers in use today, including charge-coupled device (CCD) imagers and complimentary metal oxide semi-conductor (CMOS) imagers. These imaging systems comprise an array of pixels, each of which contains a light sensitive sensor element such as a photodiode.
CMOS imagers typically utilize an array of active pixel sensors and a row of correlated double-sampling circuits or amplifiers to sample and hold the output of a given row of pixel imagers of the array. The term active pixel sensor (APS) refers to an electronic image sensor in which active devices, such as transistors, are associated with each pixel. APS devices are typically fabricated using CMOS technology.
In a CMOS imaging system, each photodiode accumulates a charge, and therefore a voltage, during the optical integration period, in accordance with the light intensity reaching the photodiode. As charge accumulates, the photodetector begins to fill. In a CMOS system, a voltage temporally stored on the capacitance of a back-biased photodiode falls in accordance with a negative charge generated by photoelectrons. The cumulative amount of charge on the photodiode at the end of the integration period is the pixel value for that pixel position. If, however, a photodetector becomes full before the end of the integration period and any additional photons strike the photodetector, then no additional charge can be accumulated. Thus, for example, a very bright light applied to a photodetector can cause a photodetector to be full before the end of the integration period and thus saturate and lose information.
In CCD imaging systems, the amount of charge that may be integrated in a pixel cell is limited by the depth of the depletion well under the photogate. The depletion gate is formed by applying a potential to the photogate that repels majority carriers from the semiconductor substrate beneath the photogate. Again, as the photogate is exposed to photons and photoelectrons are generated, the depth of the well beneath the photogate decreases. As with CMOS photodiodes, if a CCD photogate is subject to bright illumination it may saturate resulting in the loss of information about relatively bright objects in the image.
U.S. Pat. No. 6,040,570 issued Mar. 21, 2000 to Levine, et al. discloses a method of operating an APS imager to avoid the saturation problem described above. According to this method, the bias potential for the imager is applied in two steps. A first potential is applied before the start of the integration period when the pixels are reset and charge is accumulated for a first subinterval of the integration period. During this first subinterval, bright areas of the image may saturate the photodetectors in parts of the imager. In a second subinterval of the integration period, the bias voltage applied to the photodiode or to the photogate is changed to increase the charge capacity of the pixels. Pixels that previously had been saturated accumulate more charge during this second subinterval, providing a charge differential relative to other pixels that had saturated during the first subinterval. The accumulated charge on each pixel at the end of the integration period is provided as the image signal for that pixel. Thus, the dynamic range of each pixel, and therefore the complete imager, is extended to provide more information per integration period.
Furthermore, U.S. Pat. No. 5,949,918 issued Sep. 7, 1999 to McCaffery discloses a method of performing image enhancement using an APS imager, a video processor and a dual-ported memory. The video processor performs a histogramming operation to creates a look-up-table based on a cumulative distribution function (CDF) for the image. This look-up table requantizes the pixel values to increase differences between closely spaced pixel values in bright and/or dark objects in the image. As the image data is received by the video processor, it is processed through the look-up-table to increase the amount of data visible on the video display no matter what the intensity of the background or foreground of the image.
It would be preferable to use both of these processes in a single chip CMOS imager to provide a low cost, low power imager.